The present invention relates to a temperature self-compensating decoupling filter for high frequency transceivers.
More precisely the invention relates to a duplexer of the aforesaid type and it comprises a pair of filters with a specular shape, each one of which comprises a plurality of resonance cavities inside of which a corresponding adjustment disc is lodged in a removable and coaxial way provided with a stem coming out of a passage hole realised at the bottom of the cavity, whereby each filter is mechanically realised by means of an assemblage of two bodies (an upper and a lower one), so that the cavities of one or the other turn out to be facing each other in a coaxial way.
The invention relates in particular, but not exclusively, a duplexer for telecommunication systems adapted to receive and to transmit radio frequency signals, and the following description is made referring to this field of application with the only scope of simplifying the exposure.
As already well known, one of the fundamental elements of a radio telecommunication system, is represented by a device called duplexer, which connects together a transmitter TX, a receiver RX and a unique input/output antenna.
Normally the transmitter TX and the receiver RX of an antenna for high frequency transmissions operate with two different frequencies. These are physically placed next to each other, but they are decoupled by means of the aforesaid duplexer.
Such duplexer comprises essentially a pair of elementary filtering units, mutually specular, which are linked through a particular T-shaped junction placed at their centre. More in particular, an antenna duplexer F for telecommunications comprises an elementary filter Frx for the receiver and an elementary filter Ftx for the transmitter, according to an essentially symmetrical structure.
In this application, each elementary filter is set up by a body provided with a predetermined number of cylindrical cavities, coupled by means of through passing connections called coupling irises. Inside of the cavities of the elementary filters suitable internally sliding discs are placed, in order to tune in frequency each resonant cavity of each filter.
The architecture of the resonant elements, understood as cavity-disc coupling, makes it possible to obtain a particular response in frequency for each elementary filter, and the coupling of two elementary filters at the sides of the T-shaped junction makes it possible to obtain a particular response in frequency for the antenna duplexer.
The response in frequency of an elementary filter, or of the antenna duplexer as a whole, is defined by a predetermined attenuation in passband and by means of a band with a resonance number equal to the number of resonant cavities.
This response in frequency has to present stable characteristics in order to stay within certain realisation specifications. For example, some international specifications have to be respected to guarantee the uniformity of the systems on the market.
But in case of variation of the temperature, the resonant cavities of the filters are subjected to an expansion (of some parts per million) and can in this way change their resonance frequency modifying consequently the response in frequency of each filter. Obviously the useful band of the filters that are used in telecommunication appliances are anyhow maintained within the band imposed by the specifications related to each specific application. This is guaranteed by suitable security margins.
But there is a limit to freedom of choice of these security margins. In particular, it is recommended to consider that outside of the band the filters have to be subject to other specifications, for example inherent to the minimum attenuation, which further influence the final band amplitude to be obtained, which therefore always turns out to be the result of a compromise.
It is therefore particularly important to be able to assign to a resonant structure of the described type a stability in temperature, since, especially in telecommunication applications, the filters are subjected to sensible temperature variations during their functioning. In fact filters are normally mounted in more complex appliances which are quite often used outdoors and therefore subject to variable meteorological conditions due to the installation site and the period of the year.
In order to obtain a good stability in temperature, these filters are usually realised by means of an iron-nickel alloy (with a percentage of nickel of 40%), called INVAR. This alloy has an extremely limited linear factor of thermic expansion, in particular of the range of 3 ppm (parts per million).
Even though satisfactory from the point of view of the invariance of the services in function of the temperature, a filter realised with INVAR is affected by big problems, as this particular alloy is extremely expensive and difficult to process with a consequent increase of machine time necessary to realised each single piece as well as with the wear and tear of the milling-machines and of the tools used during the manufacturing procedure.
The fundamental technical problem of the present invention is that of excogitating a filter for applications in telecommunications with constant performances in case of varying temperature, presenting structural and functional characteristics enabling the limitation of costs, difficulties and manufacturing times.
The idea of a solution the present invention is based on, is that to use less valuable material than the INVAR alloy, for example steel, to realise the parts setting up each elementary filter, achieving however the wanted characteristics of performance in case of some temperature variations by means of a particular structure for the resonant cavities of the filter.
In particular according to the invention, the disc for the adjustment of the frequency is not directly inserted in the body of the elementary filter through a threaded hole, but it is connected to it by an intermediate bush element. The proposed solution exploits difference of linear thermic expansion of the materials constituting the bush and the internal disc to achieve a self-compensating effect of the resonance frequency in the case of thermic expansion.
Based on this solution idea, the technical problem is solved by a filter of the previously indicated type and characterised in that to the filter at least one bush is associated having one portion engaged in the aforesaid threaded passage hole; inside of the bush is placed the stem of a disc.